1. Field of the Invention
This invention relates to an automatic focusing device adapted for a video apparatus such as a video camera, an electronic still camera or the like.
2. Description of the Related Art
Methods of varied kinds have been known for the automatic focusing action of cameras. However, an apparatus of the kind having image sensing means for obtaining a video signal by photo-electrically converting the image of an object, such as a video camera or an electronic still camera, adopts a method whereby the definition of the object image is detected from a video signal and focus is adjusted to maximize the degree of image definition.
The apparatus of this kind is generally arranged to set a focus detecting area in a part of an image sensing plane. A focus detecting action is performed on the object image located within this area. It has recently been proposed to shift the focus detecting area while automatically tracing (tracking) the movement of an object in an effort to enhance the performance and to increase the functions of the apparatus. Such apparatuses have been disclosed, for example, in Japanese Laid-Open Patent Application No. SHO 60-249477.
Various methods also have been proposed for tracing the movement of the object. In one of them, for example, the moving position of the object is found by detecting a feature point of the object image within the focus detecting area. Then, the focus detecting area is newly set in such a way as to have the new position of the object approximately in the central part of the new area, so that a focusing action can be continuously performed on a moving object.
However, in the case of the apparatus of the above-stated kind, it is hardly possible to ensure a sufficient degree of detection accuracy as the focus detecting area must be adjusted to the moving position of the object for every field. In cases where the moving position of the object vigorously changes in particular, the focus detecting area also must be changed too much for adequate focus detection. Then, in a case where the focus detecting area is arranged to be displayed on an electronic viewfinder or the like, picture quality tends to degrade.
Previous patents and patent applications related to the present application include among others: U.S. Pat. No. 4,872,058 and U.S. patent applications Ser. Nos. 737,163 filed on May 23, 1985; 154,078 filed on Feb. 9, 1988; 237,511 filed on Aug. 26, 1988; and 240,915 filed on Sep. 6, 1988.